I would not worry too much about alternate treasures. First, you can define undefined:=0. Second, most alternate Treasures have an defined value before playing, it's just lower as that before playing.
Venture, Quarry, Talisman are 1, PhilStone can be argued to be 0 anyway before playing depending on how you read it.
The dangerous one is Fool's Gold:
If this is the first time you played a Fool’s Gold this turn, this is worth $1, otherwise it’s worth $4.
It has a defined value of you discard it, but it's ALWAYS $4.
For the cost-reduction cases: I also wouldn't care too much on KC-cost reduction-Conscription: Yeah, if you KCed some Bridges something strange might happen... If I KCed some Bridges I buy 8 Provinces, I don't care which edge-cases might also be triggered. So now that promotes the problem to Highway, and it's certainly milder than buying 8 Provinces.
So the only "real" thing I would worry about is playing 5 Highways, that's quite easy, it's a lot easier than playing 5 Bridges and 5 Highways alone don't do much without other support.
:e Idea: Use the Crossroad-mechanic? "If it's the first time you played Conscription this turn, +2 Actions."?
:e2 If you anyway want to get rid of the problems with the Treasures, you could do something like:
"Play any number of treasure cards from your hand. You may buy an action card from the supply, putting it into your hand.
Discard any treasure you have in play"
:e3 This now has some additional synergy with Black Market in drawing decks, but I don't think that's an huge issue...
:e4 And Mandarin/Mint-buys
:e5 If there's still problem with that, I don't see that the discarding is an really important part of the card. Yeah, if you draw you're whole deck you can gain an extra card without spending "real" money, but the possibility to gain a card in hand is different and interesting enough that maybe you should focus on getting this working...